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Tropical Storm HANNA (Text)

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TROPICAL STORM HANNA DISCUSSION NUMBER  30
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082008
500 AM EDT THU SEP 04 2008
 
LOOKS CAN CERTAINLY BE DECEIVING.  HANNA HAS NOT BEEN PRODUCING VERY
MUCH DEEP CONVECTION SINCE AN EARLIER STRONG BURST DIED AFTER ABOUT
04Z...AS THE UPPER-LEVEL LOW TO ITS WEST CONTINUES TO IMPOSE STRONG
SOUTHERLY SHEAR.  NEVERTHELESS...HANNA HAS A LARGE AND ROBUST
CIRCULATION...AND A NOAA RECONNAISSANCE AIRCRAFT HAS RECENTLY
REPORTED A MINIMUM CENTRAL PRESSURE OF 990 MB.  FLIGHT-LEVEL WINDS
AT 850 MB HAVE BEEN AS STRONG AS 65 KT...AND NUMEROUS SFMR VALUES
OF 55-60 KT HAVE BEEN RETRIEVED.  I HAVE NO GOOD REASON TO DISCOUNT
THE SFMR WINDS...SO THE INITIAL INTENSITY IS INCREASED TO 60 KT. 
GIVEN THAT THE SOUTHERLY SHEAR IS FORECAST BY THE DYNAMICAL MODELS
TO PERSIST OVER HANNA EVEN AS IT GAINS LATITUDE DURING THE NEXT
COUPLE OF DAYS...STRENGTHENING WOULD AT FIRST GLANCE SEEM UNLIKELY. 
CONSISTENT WITH THAT THINKING...THE GFDL AND HWRF SHOW LITTLE
CHANGE IN STRENGTH OR EVEN SOME WEAKENING...EXCEPT MUCH LATER WHEN
HANNA BECOMES AN EXTRATROPICAL CYCLONE.  ON THE OTHER HAND...THE
SHIPS AND LGEM...DESPITE DIAGNOSING LITTLE CHANGE IN
SHEAR...FORECAST A LITTLE STRENGTHENING TO HURRICANE STATUS.  A
CONSENSUS OF THESE VARIOUS MODELS WOULD YIELD A FLAT 60 KT
INTENSITY FORECAST UNTIL HANNA PASSES NEAR OR OVER THE
CAROLINAS...BUT FOR CONTINUITY AND SINCE HANNA IS ALREADY PRODUCING
WINDS JUST SHY OF HURRICANE STRENGTH....65 KT IS STILL SHOWN
EXPLICITLY IN THE FORECAST.  GRADUAL WEAKENING IS THEN FORECAST AS
HANNA ACCELERATES NORTHEASTWARD AND BECOMES EXTRATROPICAL.

DESPITE HAVING AIRCRAFT FIXES...THE INITIAL MOTION IS HARD TO
GAGE...PROBABLY DUE TO THE SPRAWLING NATURE OF THE
CIRCULATION...BUT THE INITIAL MOTION ESTIMATE IS 320/10.  HANNA
SHOULD ROUND THE BUILDING WESTERN ATLANTIC RIDGE AND ROUGHLY HUG
THE EAST COAST OF THE UNITED STATES...BUT PASSING NEAR OR OVER THE
COAST OF THE CAROLINAS ALONG THE WAY.  PERHAPS DUE IN PART TO THE
BENEFIT OF DATA FROM THE NOAA G-IV JET MISSION LAST NIGHT...THE
LATEST MODEL TRACKS HAVE COME INTO A LITTLE TIGHTER AGREEMENT...AND
IN GENERAL THEY HAVE SHIFTED SLIGHTLY TO THE LEFT.  THE NEW TRACK
FORECAST IS SHIFTED JUST SLIGHTLY LEFT OF THE PREVIOUS ONE...TO BE
CLOSER TO THE CONSENSUS.  AS ALWAYS...ONE SHOULD NOT FOCUS ON THE
EXACT FORECAST OF THE CENTER...ESPECIALLY SINCE MUCH OF THE WEATHER
ASSOCIATED WITH HANNA IS WELL-REMOVED FROM THE CENTER.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      04/0900Z 24.0N  72.8W    60 KT
 12HR VT     04/1800Z 25.3N  74.1W    60 KT
 24HR VT     05/0600Z 27.5N  76.6W    60 KT
 36HR VT     05/1800Z 30.3N  78.4W    65 KT
 48HR VT     06/0600Z 33.5N  78.4W    65 KT
 72HR VT     07/0600Z 41.0N  72.5W    50 KT...INLAND
 96HR VT     08/0600Z 48.5N  59.0W    45 KT...EXTRATROPICAL
120HR VT     09/0600Z 51.5N  41.0W    45 KT...EXTRATROPICAL
 
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FORECASTER KNABB
 
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